Added login endpoint

Started adding endpoint to login users

Repositories/UsersRepository.cs

@@ -30,6 +30,20 @@ public class UsersRepository
     #endregion
 
     #region Metods
+    public Tuple<bool, User> Exists(User user)
+    {
+        var allUsers = this.RetrieveAllUsers();
+        var result = allUsers.FirstOrDefault(ea => ea.Username!.Equals(user.Username));
+
+        if (result != null)
+        {
+            return new Tuple<bool, User>(true, result!);
+        }
+        else
+        {
+            return new Tuple<bool, User>(false, new User());
+        }
+    }
     public void CreateUser(User user)
     {
         this._bookCollection.InsertOne(user);
@@ -92,6 +106,11 @@ public class SaltRepository
         return this._bookCollection.Find(_ => true).ToList();
     }
 
+    public Salt Retrieve(string userId)
+    {
+        return this._bookCollection.Find(s => s.UserId.Equals(userId)).FirstOrDefault();
+    }
+
     public void Update(Salt salt)
     {
         var filter = Builders<Salt>.Filter.Eq(u => u.Id, salt.Id);